1. Field of the Invention
The present invention relates to power supplies for electronic devices. More particularly, the present invention relates to an uninterruptible power supply to provide power to critical equipment.
2. Discussion of the Related Art
Uninterruptible power supplies (UPSs) provide power to critical equipment that cannot experience any break in service. In other words, even a short duration brownout or blackout is unacceptable. Examples of such equipment include computer servers, computer networks, telecommunication electronics, medical devices, security networks, and the like. Regulated power is available no matter the status of the power supply.
Isolated power is important for these devices so that the input power is isolated from the output power. In short, UPSs use an isolation transformer to provide clean power to the device. An isolation transformer may have the same output voltage as input voltage. Isolation of the input and output power also prevents mutual interference, and may be required under certain conditions.
A device or equipment using a conventional UPS may include two modes of operation. First is an online mode using alternating current (AC) power applied to the primary winding. The online mode converts DC power that is rectified from an AC power input. The output power from the UPS may be reduced because of loss in the converting process.
The other mode may be called a bypass mode. If the online inverter circuit fails to provide power to the secondary winding, or load side, then a bypass circuit ensures power is provided from the power supply. The bypass mode can be implemented in two ways. One bypass mode configures the bypass power directly to the output power, thereby “bypassing” the transformer entirely. This mode does not isolate the input bypass power from the output power. If the bypass power is connected directly to the output, then any power spikes will be transferred to the output side of the power supply because the bypass is not isolated.
The other mode for providing bypass power uses a switch to apply the power to the primary winding of the transformer. Drawbacks of this configuration include a voltage interruption during the switchover of power, the output of the inverter circuit needs to be the same voltage as the input, and pulse-width modulation limitations that raise the possibility of distortion, which makes it difficult to compensate the voltage to keep the output of the inverter circuit the same. In terms of supplying uninterrupted power, a transient timing issue exists as the power supply switches from online inverter mode to bypass mode. Further, the bypass and online inverter circuits are connected to the same winding. Thus, the bypass voltage needs to be the same as the output voltage. This aspect results in unregulated voltage going to the primary winding and no adjustment available for the output voltage.
In summary, measures exist to provide uninterruptible power to devices and equipment. These measures, however, reduce the effectiveness of the power supply to provide isolated power, or result in temporary loss of power when switching to a bypass mode. Thus, these approaches fail to provide totally isolated output power in an uninterrupted manner.